Numerous processes are known in the art for the liquefaction of atmospheric gases; unfortunately, these processes tend to be energy intensive. In an effort to reduce the production costs associated with the manufacture of liquid atmospheric gases, a more efficient means of liquefaction is necessary. Past process designs have fallen into two major categories: high pressure and low pressure recycle systems.
High pressure recycle systems generally utilize nitrogen as the working fluid. These systems are characterized by operating pressures up to 3000 psia, which necessitates the use of reciprocating compression and expansion machinery. Although these systems achieve a high degree of thermodynamic efficiency, capital costs of machinery, exchangers and piping (due to high operating pressures) are greatly increased.
Low pressure recycle systems generally utilize nitrogen or air as the working fluid. These processes, because of the limited working pressures (approximately 700 psig), require lower capital costs in heat transfer and compression equipment. The machinery is often more reliable, since centrifugal compressors and expanders can be used; however, thermodynamic efficiency suffers at the lower operating pressures.
Specific examples of the preceding are as follow:
U.S. Pat. No. 4,638,639 discloses a process for liquefying a permanent gas stream which includes the steps of reducing the temperature of the permanent gas stream at elevated pressures to below its critical pressure and performing at least two working fluid cycles to provide at least part of the refrigeration necessary to reduce the temperature of the permanent gas to below its critical temperature. Each working fluid cycle comprises work-expanding the cooled working fluid in countercurrent heat exchange with the permanent gas stream and with the working fluid being cooled, refrigeration thereby being provided for the permanent gas stream. In at least one working fluid cycle, work-expanding working fluid is brought into countercurrent heat exchange relationship with the permanent gas at a temperature below the critical temperature of the permanent gas and in the or each such cycle on completion of the work expansion the working fluid is at a pressure of at least 10 atmospheres (147 psi).
U.S. Pat. No. 4,189,930 discloses a method of obtaining refrigeration at a cryogenic level comprising a gaseous fluid fed in the form of an incoming stream to sustain a refrigeration load. The incoming stream is step-wise cooled and expanded with liquefaction. The liquid fluid formed is used to sustain a refrigeration load, evaporating as a consequence, and the vapor constitutes a return stream which is adiabatically compressed so as to attain a temperature close to the temperature of the incoming stream before the liquefaction thereof.
U.S. Pat. No. 4,169,361 discloses a process wherein cold is generated by compressing a refrigerant and expanding the refrigerant isentropically in a nozzle. At least a part of the expanded refrigerant is passed in indirect heat exchange with the portion of the refrigerant prior to expansion. An expansion machine can be used to work-expand a portion of the compressed refrigerant with the expanded gas returned to the compressor. The balance of the compressed stream is expanded in the nozzle.
U.S. Pat. No. 4,099,945 discloses an improvement to a process for the fractionation of air. In the process, air is subjected to rectification in a high pressure column and a low pressure column, wherein in a liquefaction cycle, nitrogen is withdrawn in the gaseous phase from the top of the high pressure column and is liquefied by heating, compression, recooling and expansion and is recycled as liquid to the high pressure column. Also, wherein a gas, e.g., air, is withdrawn from the high pressure column, is preheated and is then expanded through a low-pressure expansion turbine. The improvement disclosed comprises cooling the gas expanded in the low pressure expansion turbine in indirect heat exchange with at least a portion of the nitrogen which is heated in the liquefaction cycle.
U.S. Pat. No. 3,605,422 discloses a process for the separation of a gas mixture under low pressure into components by a low temperature fractionating operation including an integrated refrigeration system which increases the liquid producing capabilities of the process for producing relatively large quantities of high purity products in the liquid phase without decreasing efficiency of the fractionating or sacrificing purity or yield of desired products.
U.S. Pat. No. 3,285,028 discloses refrigeration methods, more particularly of the type in which a normally gaseous fluid is expanded to produce refrigeration and the expanded fluid is passed in heat exchange with the higher pressure fluid so as to warm the former and cool the latter thereby to conserve refrigeration.